Recently, Micro Electro Mechanical Systems (MEMS) devices are extensively used in motion sensing and actuating applications, such as accelerometers, gyroscopes, or micro scanning minors which are composed of a plurality of micro structures with a variety of design types, wherein the micro scanning minors are widely applicable to light reflection applications such as laser printing, image projection, head-mounted display and head-up display.
FIG. 1A is a schematic three-dimensional view of conventional torsional oscillation structure of micro actuator 100. The micro actuator 100 includes an oscillation body 102 and a pair of torsion bars 104 connected to the both sides of the oscillation body 102. A thin film is typically formed on the oscillation body 102 to increase the reflectivity of an incident light. The oscillation body 102 is connected to the supporting base (not shown) by way of the torsion bars 104. When a periodical torque applied to the actuator, the oscillation body 102 performs a reciprocal motion about the torsion bars 104 relative to the supporting base. If an incident light beam projects on the oscillation body 102, the reflected path of the incident light beam is changed according to the oscillation status of the oscillation body 102. A two-dimensional image may be formed by scanning incident light beam reflected by using two one-axial micro actuators or one two-axial micro actuator. However, the mass and the inertia of the oscillation body 102 exerted on the oscillation body 102 during the oscillation motion will cause deformation of the oscillation body 102 and resulting in a deformed reflective surface. If the peak to valley deformation measured on the reflective surface of the oscillation body 102 is too severe, for example exceeding one eighth of the wavelength of the incident light beam, the projected image quality will deteriorate due to divergence of the light beam after reflection. The peak to valley deformation of the oscillation body 102 is defined as dynamic deformation.
When the oscillation body 102 rotates about the axis Y by way of torsion bars 104, the dynamic deformation can be represented by the following equation (1):δ ∝ {[ρ*(1−υ2)*(2 πf)2*θd5*α]/[E*t2]}  (1)
where the dynamic deformation “δ” is defined as the maximum dynamic deformation value of oscillation body 102 and is related to the following parameters: “ρ” defining the material density of the oscillation body 102, Poisson's ratio “υ” defining a ratio of transverse to axial strain of the oscillation body, “f” defining a vibration frequency of the oscillation body 102, “θ” defining the maximum oscillation angle of the oscillation body 102, “d” defining the length from the edge of the oscillation body 102 to the rotation axis 103, “α” defining circular shape coefficient of the oscillation body 102, “E” defining Young's modulus of the oscillation body 102, and “t” defining the thickness of the oscillation body 102.
FIG. 1B is a schematic view of the dynamic deformation in the oscillation structure of micro actuator 100 shown in FIG. 1A. When the torque 106 formed by the actuator exerts on the torsion bars 104, the oscillation body 102 rotates along the Y-axis and deforms due to its own mass and inertia. In FIG. 1B, line “CA” is a side view of profile if the oscillation body 102 rotates as an ideal rigid body about the Y-axis at an angle “θ”, curve “CB” is a side view of the deformation profile when the oscillation body 102 rotates about the Y-axis at an angle “θ”, and “δ” is defined as the maximum dynamic deformation value of oscillation body 102. The maximum dynamic deformation “δ” is proportional to the mass, oscillation frequency, oscillation angle and the length measured from the edge of the oscillation body 102 to the rotation axis, and is negatively proportional to the thickness of the oscillation body 102.
Referring to FIG. 1A, when the torque formed by the actuator 100 exerts on the torsion bars 104 or the oscillation body 102, the oscillation body 102 rotates until the oscillation body 102 reaches a maximum rotation angle, and the energy stored in the torsion bars 104 is in the form of elastic potential energy. The torque may be released or reversed in direction such that the elastic potential energy is converted into kinetic energy to drive the oscillation body 102 to form oscillation motion. The kinetic energy is transmitted through the torsion bars 104 and the first region RA1 of the oscillation body 102 and cause deformation in the first region RA1. Since the second region RA2 of the oscillation body 102 is distant from the rotational axis at a maximum distance, the dynamic deformation of the second region RA2 may be in a reverse direction due to the inertia of the oscillation body 102.
In FIG. 1C, a support frame 108 is disposed in the periphery of the oscillation body 102 for reducing the dynamic deformation of the oscillation body. However, because the connection part 110 which connects the frame 108 to the oscillation body 102 is in line with the torsion bars 104 along rotation axis 105, energy from the torsion bars 104 is directly transmitted to the oscillation body 102 through the torsion bars 104 and the connection part 110 such that the dynamic deformation of the oscillation body 102 cannot be efficiently reduced. Another technique of reducing dynamic deformation as shown in FIG. 1D, an additional support structure 112 with specific shape is disposed under the oscillation body 102 to reinforce the structure and further reduce the dynamic deformation. However, the additional structure complicates the micro actuator 100 manufacturing processes and may increase manufacturing difficulty and cost. Consequently, it would be desire to develop a novel oscillation structure of micro actuator to overcome the aforementioned disadvantages.